Experimental Research On The Disturbance Between Hydraulic Fractures And The Law Of Fracture Propagation

Hydraulic fracturing is an important method for reforming coal reservoirs,improving coal seam permeability,eliminating outbursts,and preventing rock bursts.Hydraulic fracturing is affected by in-situ stress and construction parameters,making it difficult to control the direction of fracture propagation,and pressure relief "blind areas" appear in the fracturing area,which brings safety hazards to underground coal mining.How to obtain the propagation law of hydraulic fracturing fractures in coal reservoirs,and use the law to make the fracture propagation direction controllable,so as to form a uniform and complex fracture network is a key problem to be solved urgently.In this paper,through physical experiments and numerical simulation methods,the law of fracture initiation and propagation of hydraulic fracturing is studied.By constructing a physical model of coal seam hydraulic fracturing,the influence of different factors on the deflection direction of double fractures under the conditions of simultaneous expansion and sequential expansion is investigated.Combining finite element and discrete element numerical simulation software to carry out numerical analysis on the factors affecting crack propagation,revealing the control mechanism of coal seam hydraulic crack propagation.The main results are as follows:(1)By constructing a physical experimental model of simultaneous fracture expansion and sequential expansion,the influence mechanism of factors such as fracture spacing,horizontal stress difference and pumping displacement on the initiation and expansion of hydraulic fracturing is investigated.The physical experiment results show that under the condition of simultaneous expansion of multiple fractures,the distribution of fracturing fluid displacement at different fractures is uneven,and the degree of fracture expansion is not uniform;In the case of sequential fracture fracturing,the induced stress field is difficult to change the original stress field under high ground stress difference,the fracture deflection angle is reduced,and it is easy to form a fracture parallel to the direction of the maximum principal stress.The small crack spacing makes the adjacent cracks in the high induced stress area,which increases the degree of stress interference and crack deflection.The large pumping displacement increases the hydrostatic pressure inside the fracture and the angle and degree of fracture deflection.(2)Considering the combined effect of hydraulic fracture-induced stress and ground stress,a theoretical model of inter-fracture interference under the coupling action of stress field and seepage field is established,and the mechanism of inter-fracture stress interference is revealed.The finite element numerical simulation results show that the crack propagation will form a stress concentration zone around the fracture,and the initiation pressure of the new fracture within the stress concentration zone will increase.The pressure relief zones formed by adjacent fractures can influence and communicate with each other,which helps to form a large-scale fracture network and improve the fracturing effect.Different factors have different influences on the interference between the slits,and the specific order is "horizontal stress difference>slit spacing> pumping displacement".The greater the horizontal stress difference,the weaker the inter-slit interference effect.(3)The discrete element numerical simulation software is used to statistically analyze the fracture trend after coal body rupture under different fracture spacing conditions.The results show that the fracture propagation pattern is affected by many factors.When the principal stress direction is parallel to the drilling direction,the resulting fracture pattern is single,the inter-fracture interference effect is weak,and the fracturing range is small;when the gap between the cracks is too small,the interference between the cracks near the crack initiation point is serious,and the phenomenon of "cracking" between different crack initiation points is obvious.Based on the conclusions drawn from physical experiments and numerical simulation data analysis,taking long borehole hydraulic fracturing as an example,the engineering application of hydraulic fracturing technology under different conditions is discussed,and it provides guidance for the construction of long borehole hydraulic fracturing in coal seams.